mtd: nand: convert printk() to pr_*()

Instead of directly calling printk, it's simpler to use the built-in
pr_* functions. This shortens code and allows easy customization through
the definition of a pr_fmt() macro (not used currently). Ideally, we
could implement much of this with dev_* functions, but the MTD subsystem
does not necessarily register all its master `mtd_info.dev` device, so
we cannot use dev_* consistently. See:
http://lists.infradead.org/pipermail/linux-mtd/2011-July/036950.html



Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>